The present invention generally relates to semiconductor devices and more particularly to a semiconductor material having a composite structure for reducing the scattering of carriers by optical phonons.
Compound semiconductor materials such as GaAs or a mixed crystal thereof are characterized by the small effective mass and are used extensively in various high-speed semiconductor devices such as HEMT or MESFET. In order to exploit the advantageous property of the compound semiconductor material and to realize much faster devices, intensive and extensive efforts are made worldwide.
On the other hand, these compound semiconductor materials, generally formed of a group 11 or III element and a group V or VI element, are polar compounds in nature and show a polarity in the crystal structure of the material- Associated with the polarity, optical phonons induce a macroscopic electric field, and such a macroscopic electric field tends to cause scattering of carriers when the material is used for an active part of a semiconductor device. This problem of scattering by optical phonons becomes particularly conspicuous in the high-speed devices such as a HEMT where a large acceleration of carriers occurs. When the scattering occurs, the carrier velocity tends to be saturated at a relatively low level because of the scattering by the optical phonons even when there is a large acceleration of carriers in the channel region of the device.
In order to overcome the problem of scattering of carriers by the optical phonons, there is a proposal to sandwich a layer of polar compound semiconductor material by a pair of layers of a homopolar material such that the oscillation of electric field, which is induced by the optical phonons in the compound semiconductor material, is reduced at the homopolar material layer (Mori, N. and Ando, T., Phys. Rev. vol. 40, no.9, pp.6175-6188, 1989). In this prior art reference, the damping rate of poiarons is studied theoretically for a structure wherein an InAs layer is sandwiched by a pair of Ge layers. It was shown that the damping of polarons is substantially reduced in this sandwich structure particularly when the thickness of the InAs layer is reduced, indicating the reduction of carrier scattering by the optical phonons as a result of use of the homopolar material. On the other hand, the sandwich structure of InAs and Ge is a mere hypothetical or theoretical structure that the structure cannot be fabricated actually because of the large discrepancy in the lattice constant between InAs and Ge.
As a combination of the polar material and the homopolar material that satisfies the matching of lattice constant, one can conceive use of a sandwich structure of GaAs and Ge wherein a layer of GaAs is sandwiched by a pair of Ge layers. In this combination of materials, the difference in the lattice constant is within 0.1%. In this sandwich system, however, there arises a problem in that the GaAs layer does not form a potential well but a potential barrier, and carriers are accumulated in the Ge layer instead of the GaAs layer.
FIG. 1 shows the band diagram of the foregoing sandwich structure that includes. A GaAs layer 1 sandwiched by a pair of Ge layers 2 in the thermal equilibrium state. In FIG. 1, the continuous line represents the conduction band for the .GAMMA. valley while the broken line represents the conduction band for the L valley. As can be seen clearly, the GaAs layer 1 forms a potential barrier with respect to the Ge layer 2, and the electrons are accumulated in the Ge layer 2 rather than in the GaAs layer 1.
FIGS.2(A) and 2(B) are the band diagrams showing the well known band structure of Ge and GaAs respectively. As can be seen in FIG. 2(A), the conduction band of Ge has the lowest energy state at the L valley, and the electrons usually occupy this state. On the other hand, the electrons in this state exhibit a relatively large effective mass (m.sup.*.sub.1 =1.6 m.sub.o, m.sup.*.sub.t =0.082 m.sub.0 ; m.sub.0 represents the mass of electrons; m.sup.*.sub.1 and m.sup.*.sub.t represent respectively the effective mass in the longitudinal direction and in the lateral direction of the energy ellipsoid that represents the iso-energy surface in the wave vector space), and because this, the Ge crystal cannot transport the electrons at a high speed. On the other hand, there exists another valley designated as .GAMMA. valley in the conduction band of Ge at an energy level higher than the L valley by about 100 meV. At the .GAMMA. valley of Ge, it is known that the electrons exhibit a very small effective mass of about 0.042m.sub.o, which is even smaller then the effective mass of electrons in the GaAs crystal. On the other hand, the .GAMMA. valley of Ge is not the stable state and the conventional device could not use this preferable feature of Ge crystal.
In GaAs, on the other hand, the lowest energy state of the conduction band is realized at the .GAMMA. valley, and the electrons show a very small effective mass of 0.067m.sub.o in correspondence to the .GAMMA. valley. Because of this advantageous feature, the GaAs crystal has been used extensively in the conventional high speed semiconductor devices. Further, there exists an L valley in the conduction band of GaAs at an energy level higher than the .GAMMA. valley by about 300 meV.
Referring to the structure of FIG. 1 again, it will be noted that the .GAMMA. valley of the GaAs layer 1 acts as the potential barrier for the electrons that occupy the .GAMMA. valley of the Ge layer 2, while the L valley of the GaAs layer 1 acts as the potential barrier for the electrons that occupy the L valley of the Ge layer 2. In such a structure of FIG. 1, one cannot achieve the desired high speed transportation of carriers even when the scattering of the carriers by optical phonons is successfully minimized by the sandwich structure of polar compound and homopolar compound, because of the slow transport of carriers through the Ge layer 2.